Series of parallel sensing operations for multi-level cells

ABSTRACT

Disclosed herein are related to a circuit and a method of reading or sensing multiple bits of data stored by a multi-level cell. In one aspect, a first reference circuit is selected from a first set of reference circuits, and a second reference circuit is selected from a second set of reference circuits. Based at least in part on the first reference circuit and the second reference circuit, one or more bits of multiple bits of data stored by a multi-level cell can be determined. According to the determined one or more bits, a third reference circuit from the first set of reference circuits and a fourth reference circuit from the second set of reference circuits can be selected. Based at least in part on the third reference circuit and the fourth reference circuit, additional one or more bits of the multiple bits of data stored by the multi-level cell can be determined.

BACKGROUND

Developments in electronic devices, such as computers, portable devices,smart phones, internet of thing (IoT) devices, etc., have promptedincreased demands for memory devices. In general, memory devices may bevolatile memory devices and non-volatile memory devices. Volatile memorydevices can store data while power is provided, by but may lose thestored data once the power is shut off. Unlike volatile memory devices,non-volatile memory devices may retain data even after the power is shutoff, but may be slower than the volatile memory devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a diagram of a memory device including multi-level cells, inaccordance with one embodiment.

FIG. 2 is a diagram of a sense device that performs a series of parallelsensing operations, in accordance with some embodiments.

FIG. 3 is an example timing diagram of performing a series of parallelsensing operations, in accordance with some embodiments.

FIG. 4 is a flowchart of a method of reading multiple bits of datastored by a multi-level cell, in accordance with some embodiments.

FIG. 5 is an example block diagram of a computing system, in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In accordance with some embodiments, data stored by a multi-level cell(MLC) is read through a series of parallel sensing operations. In oneaspect, a first parallel sensing operation is performed through two ormore sense amplifiers to determine a first bit of multiple bits of datastored by the MLC. For example, a first reference circuit is selectedfrom a first set of reference circuits, and a second reference circuitis selected from a second set of reference circuits to determine one ormore bits (e.g., most significant bits (MSBs)) of the multiple bits ofdata stored by the MLC. According to the determined one or more bits, asecond parallel sensing operation can be performed through the two ormore sense amplifiers. For example, a third reference circuit from thefirst set of reference circuits and a fourth reference circuit from thesecond set of reference circuits can be selected. Based at least in parton the third reference circuit and the fourth reference circuit,additional one or more bits (e.g., least significant bit (LSBs)) of themultiple bits of data stored by the multi-level cell can be determined.

Advantageously, the series of parallel sensing operations disclosedherein allows sensing multiple bits of data stored by an MLC in an areaand time efficient manner. For example, serial sensing through a singlesense amplifier allows reduction in an area of the integrated circuitbut may take a long time. Meanwhile, parallel sensing through multiplesense amplifiers allows sensing in a time efficient manner, but multiplesense amplifiers may consume a large area of the integrated circuit. Bydetermining one or more bits (e.g., MSBs) of multiple bits of datastored by the MLC through a first parallel sensing operation and one ormore bits (e.g., LSBs) of the multiples bits of data stored by the MLCthrough a second parallel sensing operation based on the first parallelsensing operation, less number of comparators can be implementedcompared to a number of comparators for performing a single parallelsensing operation. Moreover, a speed of comparison can be improvedcompared to a speed of comparison for performing a serial sensingoperation. Hence, multiple bits of data stored by the MLC can be read inan area and time efficient manner.

FIG. 1 is a diagram of a memory device 100, in accordance with oneembodiment. In some embodiments, the memory device 100 includes a memorycontroller 105 and a memory array 120. The memory array 120 may includea plurality of storage units or storage circuits 125 arranged in two orthree dimensional arrays. Each storage circuit may be coupled to acorresponding word line WL and a corresponding bit line BL. The memorycontroller 105 may write data to or read data from the memory array 120according to electrical signals through word lines WL and bit lines BL.In other embodiments, the memory device 100 includes more, fewer, ordifferent components than shown in FIG. 1.

The memory array 120 is a hardware component that stores data. In oneaspect, the memory array 120 is embodied as a semiconductor memorydevice. The memory array 120 includes a plurality of storage units orstorage circuits 125. The memory array 120 includes word lines WL0, WL1. . . WLJ, each extending in a first direction (e.g., X-direction) andbit lines BL0, BL1 . . . BLK, each extending in a second direction(e.g., Y-direction). The word lines WL and the bit lines BL may beconductive metals or conductive rails. In one aspect, each storagecircuit 125 is coupled to a corresponding word line WL and acorresponding bit line BL, and can be operated according to voltages orcurrents through the corresponding word line WL and the correspondingbit line BL. In one aspect, each storage circuit 125 includes a MLC. AnMLC is a single memory cell capable of storing multiple bits of data.Examples of the storage circuit 125 include a volatile memory cell suchas a phase change random access memory (PCRAM) cell, a resistive randomaccess memory (RRAM) cell, or any memory cell that stores multiple bitsof data. In some embodiments, the memory array 120 includes additionallines (e.g., select lines, reference lines, reference control lines,power rails, etc.). Detailed descriptions on configurations andoperations of memory device 100 are provided below with respect to FIGS.2 through 5.

The memory controller 105 is a hardware component that controlsoperations of the memory array 120. In some embodiments, the memoryarray 120 includes a bit line controller 112, a word line controller114, and a timing controller 110. In one configuration, the word linecontroller 114 is a circuit that provides a voltage or a current throughone or more word lines WL of the memory array 120, and the bit linecontroller 112 is a circuit that provides or senses a voltage or currentthrough one or more bit lines BL of the memory array 120. In oneconfiguration, the timing controller 110 is a circuit that providescontrol signals or clock signals to synchronize operations of the bitline controller 112 and the word line controller 114. The bit linecontroller 112 may be coupled to bit lines BL of the memory array 120,and the word line controller 114 may be coupled to word lines WL of thememory array 120. In one example, to write data to a storage circuit125, the word line controller 114 provides a voltage or current to thestorage circuit 125 through a word line WL coupled to the storagecircuit 125, and applies a bias voltage to the storage circuit 125through a bit line BL coupled to the storage circuit 125. In oneexample, to read data from a storage circuit 125, the word linecontroller 114 provides a voltage or current to the storage circuit 125through a word line WL coupled to the storage circuit 125, and senses avoltage or current corresponding to data stored by the storage circuit125 through a bit line BL coupled to the storage circuit 125. In someembodiments, the memory controller 105 includes more, fewer, ordifferent components than shown in FIG. 1.

FIG. 2 is a diagram of a sensing device 205 that performs a series ofparallel sensing operations, in accordance with some embodiments. Insome embodiments, the sensing device 205 is part of the bit linecontroller 112 of FIG. 1. In some embodiments, the sensing device 205includes a sense controller 210, sense amplifiers 240A-240C,multiplexers 220A-220C, and reference circuits 250AA . . . 250AE, 250BA. . . 250BE, 250CA . . . 250CE. In one aspect, these components operatetogether to perform a series of parallel sensing operations to read datafrom the storage circuit 125 (also referred to as “a MLC 125” herein).In other embodiments, the sensing device 205 includes more, fewer, ordifferent components than shown in FIG. 2. For example, in someembodiments, the sensing device 205 includes more or different number ofsense amplifiers 240, multiplexers 220, and reference circuits 250 thanshown in FIG. 2.

In some embodiments, each reference circuit 250 is a circuit thatprovides a corresponding reference characteristic. An examplecharacteristic includes a discharge rate, a resistance, a voltage, etc.In some embodiments, the reference circuits 250 can be replaced by othercomponents that perform similar functions of the reference circuits 250.In one aspect, a characteristic (a discharge rate, a resistance, avoltage, etc.) of the MLC 125 can be set, programmed or configured, froma plurality of predetermined resistances, according to multiple bits ofdata stored. For example, a resistance of a memory cell between areference resistance 0^(th) R_(ref) (e.g., 400Ω) and a referenceresistance 1^(th) R_(ref) (e.g., 4 kΩ) represents [0001], a resistanceof the memory cell between the reference resistance 1^(th) R_(ref)(e.g., 4 kΩ) and a reference resistance 2^(nd) R_(ref) (e.g., 40 kΩ)represents [0010], and a resistance of the memory cell between thereference resistance 2^(nd) R_(ref) (e.g., 40 kΩ) and a referenceresistance 3^(rd) R_(ref) (e.g., 400 kΩ) represents [0011]. In someembodiments, the reference circuits 250 are set, designed or implementedwith different values of reference characteristics that can be comparedagainst the programmed characteristic of the MLC 125 to determinemultiple bits of data stored by the MLC 125. In one some embodiments,each of the reference circuits 250AA . . . 250AE, 250BA . . . 250BE,250CA . . . 250CE has a corresponding one of reference resistances0^(th) R_(ref) . . . 14^(th) R_(ref), that can be used to identify ordistinguish 16 values of 4 bit data. The reference resistances 0^(th)R_(ref) . . . 14^(th) R_(ref) may monotonically (linearly ornon-linearly) increase in that sequence. In one aspect, the referencecircuits 250AA . . . 250AE, 250BA . . . 250BE, 250CA . . . 250CE havenon-monotonically assigned reference resistances to allow efficientparallel sensing. In one configuration, the reference circuits 250AA . .. 250AE have reference resistances 3^(rd) R_(ref), 0^(th) R_(ref),4^(th) R_(ref), 8^(th) R_(ref), 12^(th) R_(ref), respectively; thereference circuits 250BA . . . 250BE have reference resistances 7^(th)R_(ref), 1^(st) R_(ref), 5^(th) R_(ref), 9^(th) R_(ref), 13^(th)R_(ref), respectively; and the reference circuits 250CA . . . 250CE havereference resistances 11^(th) R_(ref), 2^(nd) R_(ref), 6^(th) R_(ref),10^(th) R_(ref), 14^(th) R_(ref), respectively. Such arrangement ofresistances allows efficient parallel sensing operations, as describedbelow with respect to FIG. 3.

In some embodiments, each multiplexer 220 is a circuit that electricallycouples, from a corresponding subset of reference circuits 250, aselected reference circuit to a corresponding sense amplifier 240. Inother embodiments, the multiplexers 220 can be replaced by othercomponents that perform similar functions of the multiplexers 220. Inone configuration, the multiplexer 220A includes input ports coupled toa subset of reference circuits 250AA . . . 250AE, and an output portcoupled to an input port of the sense amplifier 240A. In addition, themultiplexer 220B includes input ports coupled to a subset of referencecircuits 250BA . . . 250BE, and an output port coupled to an input portof the sense amplifier 240B. In addition, the multiplexer 220C includesinput ports coupled to a subset of reference circuits 250CA . . . 250CE,and an output port coupled to an input port of the sense amplifier 240C.Moreover, each of the multiplexers 220A-220C includes a control portcoupled to the sense controller 210 to receive a control signal from thesense controller 210. In this configuration, each of the multiplexers220A-220C may electrically couple, from a corresponding subset ofreference circuits 250, a selected reference circuit to a correspondingsense amplifier 240 according to a control signal from the sensecontroller 210. For example, the multiplexer 220A electrically couplesthe reference circuit 250AA to the sense amplifier 240A, while themultiplexer 220B electrically couples the reference circuit 250BA to thesense amplifier 240B and the multiplexer 220C electrically couples thereference circuit 250CA to the sense amplifier 240C. When themultiplexer 220 electrically couples, from a subset of referencecircuits, a selected reference circuit to a corresponding senseamplifier, the remaining reference circuits of the subset of thereference circuits may be electrically decoupled from the correspondingsense amplifier. Hence, a voltage, current, or resistance at theselected sense amplifier can be provided to the corresponding senseamplifier, while voltages, currents, or resistances at the remainingsense amplifiers may not be provided to the corresponding senseamplifier.

In some embodiments, each sense amplifier 240 is a circuit that comparesa characteristic of the MLC 125 with a reference characteristic of aselected reference circuit 250 through a corresponding multiplexer 220.In other embodiments, the sense amplifier 240 can be replaced by othercomponents that perform similar functions of the sense amplifiers 220.In one configuration, each sense amplifier includes a first input port(e.g., shown as “−”) coupled to the MLC 125 through the bit line BL, asecond input port (e.g., shown as “+”) coupled to a correspondingmultiplexer 220, and an output port coupled to the sense controller 210.In one configuration, a sense amplifier 240 compares a discharge rate ofa MLC 125 with a discharge rate of the selected reference circuit 250.In one aspect, the MLC 125 can be modeled as a resistor R_(cell) with acapacitor C_(BL) corresponding to a capacitance of the bit line BL. Inone example, a voltage of the MLC 125 at the bit line BL can decrease ata rate corresponding to the resistance R_(cell) set or programmed.Similarly, a voltage of the selected reference circuit 250 can decreaseat a rate corresponding to a predetermined resistance of the selectedreference circuit 250. In one approach, the sense controller 210 maycharge or set the MLC 125 and the selected reference circuit 250 to havethe same voltage. After a predetermined time period (e.g., 10 ns), avoltage of the MLC 125 at the bit line BL and a voltage of the selectedreference circuit 250 may differ, because the MLC 125 and the selectedreference circuit 250 have different discharge rates. After thepredetermined time period, the sense amplifier 240 may detect adifference in a voltage of the MLC 125 at the bit line BL and a voltageof the selected reference circuit 250, and generate an output signalaccording to the detected difference. For example, in response to avoltage (e.g., 400 mV) of the MLC 125 being higher than a voltage (e.g.,300 mV) of the selected reference circuit 250, the MLC 125 may generatean output signal having 0V or logic [0]. Conversely, in response to avoltage (e.g., 300 mV) of the MLC 125 being lower than a voltage (e.g.,400 mV) of the selected reference circuit 250, the MLC 125 may generatean output signal having VDD or logic value [1].

In some embodiments, the sense controller 210 is a circuit thatconfigures one or more components of the sensing device 205 to perform aseries of parallel sensing operations to determine multiple bits of datastored by the MLC 125 according to outputs from the sense amplifiers240A-240C. The sense controller 210 may be embodied as a digital logiccircuit or a state machine. In other embodiments, the sense controller210 can be replaced by other components that perform similar functionsof the sense controller 210. In one configuration, the sense controller210 includes input ports coupled to outputs ports OutA, OutB, OutC ofthe sense amplifiers 240A-240C. In addition, the sense controller 210includes one or more output ports coupled to control ports of themultiplexers 220A-220C to provide control signals for one or morereference circuits 250. In this configuration, the sense controller 210can perform a first parallel sensing operation through the senseamplifiers 240A-240C to determine a value (or values) of one or morebits (e.g., MSB) of multiple bits of data stored by the MLC 125. Inaddition, the sense controller 210 can perform a second parallel sensingoperation through the sense amplifiers 240A-240C based at least in parton the one or more bits (e.g., MSB) of the multiple bits of datadetermined by the first parallel sensing operation to determine a value(or values) of one or more bits (e.g., LSB) of multiple bits of datastored by the MLC 125. Assuming for an example that the MLC 125 canstore 4 bit data [YYXX], the sense controller 210 may perform the firstparallel sensing through the sense amplifier 240A-240C to determinevalues of the two MSBs [YY]. Then, according to the MSBs [YY], the sensecontroller 210 may perform the second parallel sensing through the senseamplifier 240A-240C to determine values of the two LSBs [XX].

In some embodiments, to perform a parallel sensing operation, the sensecontroller 210 may configure each of the multiplexers 220A-220C toselect a predetermined reference circuit from a corresponding subset ofreference circuits 250, and determine one or more bits (e.g., MSB) ofdata stored by the MLC 125 according to the selected reference circuits.In one approach, the sense controller 210 configures the multiplexer220A-220C to select the reference circuits 250AA, 250BA, 250CA havingpredetermined or pre-assigned reference resistances (e.g., 3^(rd)R_(ref), 7^(th) R_(ref), 11^(th) R_(ref)) for determining the one ormore bits (e.g., MSBs). In one example, the reference resistances 3^(rd)R_(ref), 7^(th) R_(ref), 11^(th) R_(ref) are predetermined or assignedfor determining first two bits of multiple bits (e.g., [YY] of [YYXX]).For example, the MLC 125 storing any of [0000], [0001], [0010], [0011]has a resistance lower than the reference resistance 3^(rd) R_(ref); theMLC 125 storing any of [0100], [0101], [0110], [0111] has a resistancebetween than the reference resistance 3^(rd) R_(ref) and the referenceresistance 7^(th) R_(ref); the MLC 125 storing any of [1000], [1001],[1010], [1011] has a resistance between than the reference resistance7^(rd) R_(ref) and the reference resistance 11^(th) R_(ref); and the MLC125 storing any of [1100], [1101], [1110], [1111] has a resistancegreater than the reference resistance 11^(rd) R_(ref). During the firsttime period, the sense amplifiers 240A, 240B, 240C may comparecharacteristic (e.g., discharge rate) of the MLC 125 with characteristic(e.g., discharge rates) of the selected reference circuits (e.g., 250AA,250BA, 250CA), and determine a value (or values) of one or more bits(e.g., MSB) of multiple bits of data stored by the MLC 125 according tothe comparison.

In one example, a discharge rate less than a discharge rate of thereference circuit 250AA corresponds to [00XX] of data stored by the MLC125; a discharge rate between i) the discharge rate of the referencecircuit 250AA having the reference resistance 3^(rd) R_(ref) and ii) adischarge rate of the reference circuit 250BA having the referenceresistance 7^(th) R_(ref) corresponds to [01XX] of data stored by theMLC 125; a discharge rate between i) the discharge rate the referencecircuit 250BA and ii) a discharge rate of the reference circuit 250CAhaving the reference resistance 11^(th) R_(ref) corresponds to [10XX] ofdata stored by the MLC 125; and a discharge rate higher than thedischarge rate of the reference circuit 250CA corresponds to [11XX] ofdata stored by the MLC 125. In this example, the sense controller 210can determine first two MSBs according to outputs OutA, OutB, OutC fromthe sense amplifiers 240A-240C. For example, if the outputs OutA, OutB,OutC are [110] in a thermometer code (corresponding to ‘2’ in a decimalrepresentation as indicated a number of ‘1’), the sense controller 210may determine that a discharge rate of the MLC 125 is higher thandischarge rates of the reference circuits 250AA, 250BA but less than thedischarge rate of the reference circuit 250CA. Accordingly, the sensecontroller 210 may determine that first two MSBs of the MLC 125 is [10]in a binary code (corresponding to ‘2’ in the decimal representation).

In some embodiments, to perform a subsequent parallel sensing, the sensecontroller 210 may select, from each of different sets of referencecircuits, a reference circuit according to the determined one more bitsthrough the prior parallel sensing, and cause the multiplexers 220A-220Cto electrically couple the selected reference circuits to correspondingsense amplifiers 240A-240C during a second time period. For example, inresponse to [00] of two bits determined during the prior sensing, thereference circuits 250AB, 250BB, 250CB having resistances 0^(th)R_(ref), 1^(st) R_(ref), 2^(nd) R_(ref), respectively, are selected todetermine values of subsequent two bits [XX] from [00XX]; in response to[01] of two bits determined during the prior sensing, the referencecircuits 250AC, 250BC, 250CC having resistances 4^(th) R_(ref), 5^(th)R_(ref), 6^(th) R_(ref), respectively, are selected to determinesubsequent two bits [XX] from [01XX]; in response to [10] of two bitsdetermined during the prior sensing, the reference circuits 250AD,250BD, 250CD having resistances 8^(th) R_(ref), 9^(th) R_(ref), 10^(th)R_(ref), respectively, are selected to determine subsequent two bits[XX] from [10XX]; and in response to [11] of two bits determined duringthe prior sensing, the reference circuits 250AE, 250BE, 250CE havingreference resistances 12^(th) R_(ref), 13^(th)R_(ref), 14^(th) R_(ref),respectively, are selected, to determine subsequent two bits [XX] from[11XX]. Assuming for an example that two bits determined through theprior parallel sensing is [10], the sense controller 210 selects thereference circuits 250AD, 250BD, 250CD having reference resistances8^(th) R_(ref), 9^(th) R_(ref), 10^(th) R_(ref), associated with [10]for determining subsequent bits [XX] of [10XX]. Then, the sensecontroller 210 generates one or more control signals to configure themultiplexers 220A, 220B, 220C to electrically couple the selectedreference circuits 250AD, 250BD, 250CD to the sense amplifiers 240A,240B, 240C, respectively, during the second time period. The senseamplifiers 240A-240C may compare characteristics of selected referencecircuits with the characteristic of the MLC 125 during the second timeperiod. The sense controller 210 may determine one or more bits (e.g.,LSB) of multiple bits of data stored by the MLC 125 according to outputsfrom the sense amplifiers 240A-240C during the second time period in asimilar manner as determining the one or more bits (e.g., MSB) of themultiple bits of data.

Advantageously, the series of parallel sensing operations disclosedherein allows sensing multiple bits of data stored by a MLC 125 in anarea and time efficient manner. For example, serial sensing through asingle sense amplifier allows reduction in an area of the integratedcircuit, but may take a long time. Meanwhile, parallel sensing throughmultiple sense amplifiers allows sensing in a time efficient manner, butmultiple sense amplifiers may consume a large area of the integratedcircuit. By determining a value (or values) of one or more bits (e.g.,MSBs) of multiple bits (e.g., [YY] of [YYXX]) of data stored by the MLC125 through a first parallel sensing operation and determining one ormore bits (e.g., LSBs) of the multiples bits (e.g., [XX] of [YYXX]) ofdata stored by the MLC 125 through a second parallel sensing operationbased on the first parallel sensing operation, less number ofcomparators can be implemented compared to a number of comparators forperforming a single parallel sensing operation. Moreover, a speed ofcomparison can be improved compared to a speed of comparison forperforming a serial sensing operation. Hence, multiple bits of datastored by the MLC can be read in an area and time efficient manner.

FIG. 3 is an example timing diagram 300 of performing parallel sensingand serial sensing, in accordance with some embodiments. Assuming for anexample that a MLC 125 can have one of 16 predetermined resistancestates to represent 4 bit data. In one approach, to read stored datafrom the MLC 125, the MLC 125 can perform a series of parallel sensingoperations.

For the first parallel sensing operation, in one approach, the sensecontroller 210 configures the multiplexers 220A, 220B, 220C to selectpredetermined reference circuits 250AA, 250BA, 250CA having referenceresistances 3^(rd) R_(ref), 7^(th) R_(ref), and 11^(th) R_(ref). In oneexample, the reference resistances 3^(rd) R_(ref), 7^(th) R_(ref),11^(th) R_(ref) are predetermined or assigned for determining first twobits of multiple bits of data stored (e.g., [YY] of [YYXX]). During afirst time period, the multiplexer 220A can electrically couple thereference circuit 250AA to the sense amplifier 240A, the multiplexer220B can electrically couple the reference circuit 250BA to the senseamplifier 240B, and the multiplexer 220C can electrically couple thereference circuit 250CA to the sense amplifier 240C simultaneously. Thesense amplifiers 240A, 240B, 240C can compare characteristic (e.g.,voltage or discharge rate) of the MLC 125 with characteristics of thereference circuits 250AA, 250BA, 250CA, and output the comparisons, forexample, in a thermometer code. For example, if the discharge rate ofthe MLC 125 is higher than a discharge rate of the reference circuit250AA but less than discharge rates of the reference circuits 250BA,250CA, the sense amplifiers 240A, 240B, 240C can output [100], becausethe resistance of the MLC 125 is determined to be greater than thereference resistance 3^(rd) R_(ref), but less than the referenceresistances 7^(th) R_(ref), 11^(th) R_(ref). The sense controller 210can receive outputs from the sense amplifiers 240A, 240B, 240C, anddecode the outputs in the thermometer code into 2 bit binaryrepresentation, which may correspond to MSBs of data stored by the MLC125. For example, the thermometer code [100] having one ‘1’ can beconverted into a binary representation [01].

For the second parallel sensing operation, in one approach, the sensecontroller 210 selects, for each multiplexer, a corresponding referencecircuit according to the one or more determined bits in the firstparallel sensing operation. Assuming for an example that [01] isdetermined from the first parallel sensing operation, the sensecontroller 210 configures the multiplexers 220A, 220B, 220C to selectreference circuits 250AC, 250BC, 250CC corresponding to the determinedbits [01]. The reference circuits 250AC, 250BC, 250CC may have referenceresistances 4^(th) R_(ref), 5^(th) R_(ref), and 6^(th) R_(ref),respectively. The sense amplifiers 240A, 240B, 240C can comparecharacteristic (e.g., voltage or discharge rate) of the MLC 125 withcharacteristics of the reference circuits 250AC, 250BC, 250CC, andoutput the comparisons, for example, in a thermometer code. For example,if the discharge rate of the MLC 125 is higher than a discharge rate ofthe reference circuits 250AC, 250BC, but less than discharge rates ofthe reference circuit 250CC, the sense amplifiers 240A, 240B, 240C canoutput [110]. The sense controller 210 can receive outputs from thesense amplifiers 240A, 240B, 240C, and decode the outputs in thethermometer code into a 2 bit binary representation, which maycorrespond to LSBs of data stored by the MLC 125. For example, thethermometer code [110] having two ‘1’ can be converted to binaryrepresentation [10].

FIG. 4 is a flowchart of a method 400 of reading multiple bits of datastored by a multi-level cell, in accordance with some embodiments. Themethod 400 may be performed by the sensing device 205 of FIG. 2. In someembodiments, the method 400 is performed by other entities. In someembodiments, the method 400 includes more, fewer, or differentoperations than shown in FIG. 4.

In an operation 410, the sensing device 205 performs a first parallelsensing operation through sense amplifiers (e.g., 240A-240C) withreference circuits (e.g., 250AA, 250BA, 250CA) during a first timeperiod. In one approach, the sense amplifiers (e.g., 240A-240C)simultaneously compares a characteristic (e.g., a resistance, adischarge rate, a voltage, etc.) of a memory cell (e.g., MLC 125) withcharacteristics of the reference circuits (e.g., 250AA, 250BA, 250CA)during the first time period, and determine a value (or values) of oneor more bits (e.g., MSBs) of multiple bits of data stored by the memorycell (e.g., MLC 125).

In one approach, the operation 410 includes operations 412, 414. In theoperation 412, a sense controller (e.g., 210) selects reference circuits(e.g., 250AA, 250BA, 250CA) from a set of reference circuits (e.g.,250AA-250AE, 250BA-250BE, 250CA-250CE). In one aspect, each subset ofthe set of reference circuits 250 is associated with a corresponding oneof multiplexers (e.g., 220A, 220B, 220C). For example, the referencecircuits 250AA-250AE are associated with the multiplexer 220A, thereference circuits 250BA-250BE are associated with the multiplexer 220B,and the reference circuits 250CA-250CE are associated with themultiplexer 220C. In the operation 412, the sense controller (e.g., 210)may select, from each of the subsets of the set of reference circuits250, predetermined reference circuits (e.g., 250AA, 250BA, 250CA). Thesense controller (e.g., 210) may generate one or more control signals toconfigure the multiplexers 220A-220C to electrically couple the selectedreference circuits (e.g., 250AA, 250BA, 250CA) to the correspondingsense amplifiers 240A-240C, respectively, during the first time period.

In an operation 414, the sense controller (e.g., 210) determines a value(or values) of one or more bits (e.g., MSBs) of the multiple bits ofdata stored by the memory cell (e.g., MLC 125) according to thereference circuits (e.g., 250AA, 250BA, 250CA). In one approach, themultiplexers 220A-220C can electrically couple the selected referencecircuits (e.g., 250AA, 250BA, 250CA) to the sense amplifiers (e.g.,240A-240C), respectively, according to the control signal from the sensecontroller (e.g., 210). The sense amplifiers (e.g., 240A-240C) cancompare a characteristic (e.g., a resistance, a discharge rate, avoltage, etc.) of the memory cell (e.g., MLC 125) with characteristicsof the reference circuits (e.g., 250AA, 250BA, 250CA), and generateoutputs according to the comparisons during the first time period. Inone example, the outputs of the sense amplifiers (e.g., 240A-240C) arerepresented in a thermometer code. The sense controller (e.g., 210) mayreceive the outputs of the sense amplifiers (e.g., 240A-240C) in thethermometer code, and convert the received outputs into a binaryrepresentation. For example, if the outputs OutA, OutB, OutC are [110]in a thermometer code, the sense controller (e.g., 210) may determinethat a discharge rate of the MLC 125 is higher than discharge rates ofthe reference circuits 250AA, 250BA but less than the discharge rate ofthe reference circuit 250CA. Accordingly, the sense controller 210 maydetermine that first two MSBs of the MLC 125 is [10] in a binary codecorresponding to the thermometer code [110] having two ‘1’.

In an operation 430, the sensing device 205 performs a second parallelsensing operation though the sense amplifiers (e.g., 240A-240C) withdifferent referent circuits (e.g., 250AD, 250BD, 250CD) during a secondtime period. In one approach, different reference circuits (e.g., 250)are selected according to the value (or the values) of the one or morebits determined in the operation 410. The sense amplifiers (e.g.,240A-240C) can simultaneously compare the characteristic (e.g., aresistance, a discharge rate, a voltage, etc.) of the memory cell (e.g.,MLC 125) with characteristics of the different reference circuits (e.g.,250AA, 250BA, 250CA) during the second time period, and determine one ormore bits (e.g., LSBs) of the multiple bits of data stored by the memorycell (e.g., MLC 125).

In one approach, the operation 430 includes operations 432, 434. In theoperation 432, the sense controller (e.g., 210) selects differentreference circuits (e.g., 250AD, 250BD, 250CD) from a set of referencecircuits (e.g., 250AA-250AE, 250BA-250BE, 250CA-250CE), according to thedetermined one or more bits in the operation 410. For example, thereference circuits 250AB, 250BB, 250CB having resistances 0^(th)R_(ref), 1^(st) R_(ref), 2^(nd) R_(ref), respectively, correspond to[00] of two bits determined during the prior sensing; the referencecircuits 250AC, 250BC, 250CC having resistances 4^(th) R_(ref),5^(th)R_(ref), 6^(th) R_(ref), respectively, correspond to [01] of twobits determined during the prior sensing; the reference circuits 250AD,250BD, 250CD having resistances 8^(th) R_(ref), 9^(th) R_(ref), 10^(th)R_(ref), respectively, correspond to [10] of two bits determined duringthe prior sensing; and the reference circuits 250AE, 250BE, 250CE havingresistances 12^(th) R_(ref), 13^(th) R_(ref), 14^(th) R_(ref),respectively, correspond to [11] of two bits determined during the priorsensing. Assuming for an example that two bits determined in theoperation 410 is [10], the sense controller 210 selects the referencecircuits 250AD, 250BD, 250CD.

In the operation 434, the sense controller (e.g., 210) determinesadditional one or more bits of the multiple bits of data stored by thememory cell according to the different reference circuits. In oneapproach, the multiplexers 220A-220C can electrically couple thereference circuits (e.g., 250AD, 250BD, 250CD) selected in the operation432 to the sense amplifiers (e.g., 240A-240C), respectively. The senseamplifiers (e.g., 240A-240C) can compare the characteristic (e.g., aresistance, a discharge rate, a voltage, etc.) of the memory cell (e.g.,MLC 125) with characteristics of the reference circuits (e.g., 250AD,250BD, 250CD), and generate outputs according to the comparisons duringthe second time period. The sense controller (e.g., 210) may receive theoutputs of the sense amplifiers (e.g., 240A-240C) in the thermometercode, and convert the received outputs into the binary representation.In one aspect, the one or more bits determined in the operation 434corresponds to LSB of the multiple bits of data stored by the memorycell (e.g., MLC 125).

Advantageously, the series of parallel sensing operations disclosedherein allows sensing multiple bits of data stored by a MLC 125 in anarea and time efficient manner. For example, serial sensing through asingle sense amplifier allows reduction in an area of the integratedcircuit, but may take a long time. Meanwhile, parallel sensing throughmultiple sense amplifiers allows sensing in a time efficient manner, butmultiple sense amplifiers may consume a large area of the integratedcircuit. By determining one or more bits (e.g., MSBs) of multiple bitsof data stored by the MLC through a first parallel sensing operation andone or more bits (e.g., LSBs) of the multiples bits of data stored bythe MLC 125 through a second parallel sensing operation based on thefirst parallel sensing operation, less number of comparators can beimplemented compared to a number of comparators for performing a singleparallel sensing operation. Moreover, a speed of comparison can beimproved compared to a speed of comparison for performing a serialsensing operation. Hence, multiple bits of data stored by the MLC can beread in an area and time efficient manner.

Referring now to FIG. 5, an example block diagram of a computing system500 is shown, in accordance with some embodiments of the disclosure. Thecomputing system 500 may be used by a circuit or layout designer forintegrated circuit design. A “circuit” as used herein is aninterconnection of electrical components such as resistors, transistors,switches, batteries, inductors, or other types of semiconductor devicesconfigured for implementing a desired functionality. The computingsystem 500 includes a host device 505 associated with a memory device510. The host device 505 may be configured to receive input from one ormore input devices 515 and provide output to one or more output devices520. The host device 505 may be configured to communicate with thememory device 510, the input devices 515, and the output devices 520 viaappropriate interfaces 525A, 525B, and 525C, respectively. The computingsystem 500 may be implemented in a variety of computing devices such ascomputers (e.g., desktop, laptop, servers, data centers, etc.), tablets,personal digital assistants, mobile devices, other handheld or portabledevices, or any other computing unit suitable for performing schematicdesign and/or layout design using the host device 505.

The input devices 515 may include any of a variety of input technologiessuch as a keyboard, stylus, touch screen, mouse, track ball, keypad,microphone, voice recognition, motion recognition, remote controllers,input ports, one or more buttons, dials, joysticks, and any other inputperipheral that is associated with the host device 505 and that allowsan external source, such as a user (e.g., a circuit or layout designer),to enter information (e.g., data) into the host device and sendinstructions to the host device. Similarly, the output devices 520 mayinclude a variety of output technologies such as external memories,printers, speakers, displays, microphones, light emitting diodes,headphones, video devices, and any other output peripherals that areconfigured to receive information (e.g., data) from the host device 505.The “data” that is either input into the host device 505 and/or outputfrom the host device may include any of a variety of textual data,circuit data, signal data, semiconductor device data, graphical data,combinations thereof, or other types of analog and/or digital data thatis suitable for processing using the computing system 500.

The host device 505 includes or is associated with one or moreprocessing units/processors, such as Central Processing Unit (“CPU”)cores 530A-530N. The CPU cores 530A-530N may be implemented as anApplication Specific Integrated Circuit (“ASIC”), Field ProgrammableGate Array (“FPGA”), or any other type of processing unit. Each of theCPU cores 530A-530N may be configured to execute instructions forrunning one or more applications of the host device 505. In someembodiments, the instructions and data to run the one or moreapplications may be stored within the memory device 510. The host device505 may also be configured to store the results of running the one ormore applications within the memory device 510. Thus, the host device505 may be configured to request the memory device 510 to perform avariety of operations. For example, the host device 505 may request thememory device 510 to read data, write data, update or delete data,and/or perform management or other operations. One such application thatthe host device 505 may be configured to run may be a standard cellapplication 535. The standard cell application 535 may be part of acomputer aided design or electronic design automation software suitethat may be used by a user of the host device 505 to use, create, ormodify a standard cell of a circuit. In some embodiments, theinstructions to execute or run the standard cell application 535 may bestored within the memory device 510. The standard cell application 535may be executed by one or more of the CPU cores 530A-530N using theinstructions associated with the standard cell application from thememory device 510. In one example, the standard cell application 535allows a user to utilize pre-generated schematic and/or layout designsof the memory device 100 or a portion of the memory device 100 to aidintegrated circuit design. After the layout design of the integratedcircuit is complete, multiples of the integrated circuit, for example,including the memory device 100 or a portion of the memory device 100can be fabricated according to the layout design by a fabricationfacility.

Referring still to FIG. 5, the memory device 510 includes a memorycontroller 540 that is configured to read data from or write data to amemory array 545. The memory array 545 may include a variety of volatileand/or non-volatile memories. For example, in some embodiments, thememory array 545 may include NAND flash memory cores. In otherembodiments, the memory array 545 may include NOR flash memory cores,Static Random Access Memory (SRAM) cores, Dynamic Random Access Memory(DRAM) cores, Magnetoresistive Random Access Memory (MRAM) cores, PhaseChange Memory (PCM) cores, Resistive Random Access Memory (ReRAM) cores,3D XPoint memory cores, ferroelectric random-access memory (FeRAM)cores, and other types of memory cores that are suitable for use withinthe memory array. The memories within the memory array 545 may beindividually and independently controlled by the memory controller 540.In other words, the memory controller 540 may be configured tocommunicate with each memory within the memory array 545 individuallyand independently. By communicating with the memory array 545, thememory controller 540 may be configured to read data from or write datato the memory array in response to instructions received from the hostdevice 505. Although shown as being part of the memory device 510, insome embodiments, the memory controller 540 may be part of the hostdevice 505 or part of another component of the computing system 500 andassociated with the memory device. The memory controller 540 may beimplemented as a logic circuit in either software, hardware, firmware,or combination thereof to perform the functions described herein. Forexample, in some embodiments, the memory controller 540 may beconfigured to retrieve the instructions associated with the standardcell application 535 stored in the memory array 545 of the memory device510 upon receiving a request from the host device 505.

It is to be understood that only some components of the computing system500 are shown and described in FIG. 5. However, the computing system 500may include other components such as various batteries and powersources, networking interfaces, routers, switches, external memorysystems, controllers, etc. Generally speaking, the computing system 500may include any of a variety of hardware, software, and/or firmwarecomponents that are needed or considered desirable in performing thefunctions described herein. Similarly, the host device 505, the inputdevices 515, the output devices 520, and the memory device 510 includingthe memory controller 540 and the memory array 545 may include otherhardware, software, and/or firmware components that are considerednecessary or desirable in performing the functions described herein.

One aspect of this description relates to a memory device. In someembodiments, the memory device includes a multi-level cell storingmultiple bits of data. In some embodiments, the memory device includes asensing device coupled to the multi-level cell. In some embodiments, thesensing device includes a first sense amplifier having a first inputport coupled to the multi-level cell. In some embodiments, the sensingdevice includes a first multiplexer coupled to a second input port ofthe first sense amplifier. In some embodiments, the sensing deviceincludes a second sense amplifier having a first input port coupled tothe multi-level cell. In some embodiments, the method sensing deviceincludes a second multiplexer coupled to a second input port of thesecond sense amplifier.

One aspect of this description relates to a memory device. In someembodiments, the memory device includes a multi-level cell storingmultiple bits of data. In some embodiments, the memory device includes asensing device coupled to the multi-level cell. In some embodiments, thesensing device includes a set of sense amplifiers, each of the set ofsense amplifiers including a first input port and a second input port,the first input port coupled to the multi-level cell. In someembodiments, the sensing device includes a set of reference circuits,and a set of multiplexers. Each of the set of multiplexers may becoupled between the second input port of a corresponding one of the setof sense amplifiers and a corresponding subset of the set of referencecircuits.

One aspect of this description relates to a method of reading datastored by a multi-level cell. In some embodiments, the method includesselecting, a first reference circuit from a first set of referencecircuits. In some embodiments, the method includes selecting a secondreference circuit from a second set of reference circuits. In someembodiments, the method includes determining one or more bits ofmultiple bits of data stored by a multi-level cell, based at least inpart on the first reference circuit and the second reference circuit. Insome embodiments, the method includes selecting a third referencecircuit from the first set of reference circuits, according to thedetermined one or more bits. In some embodiments, the method includesselecting a fourth reference circuit from the second set of referencecircuits, according to the determined one or more bits. In someembodiments, the method includes determining additional one or more bitsof the multiple bits of data stored by the multi-level cell, based atleast in part on the third reference circuit and the fourth referencecircuit.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A memory device comprising: a multi-level cellstoring multiple bits of data; and a sensing device coupled to themulti-level cell, the sensing device including: a first sense amplifierhaving a first input port coupled to the multi-level cell, a firstmultiplexer coupled to a second input port of the first sense amplifier,a second sense amplifier having a first input port coupled to themulti-level cell, and a second multiplexer coupled to a second inputport of the second sense amplifier.
 2. The memory device of claim 1,wherein the first multiplexer is coupled to a first set of referencecircuits, and wherein the second multiplexer is coupled to a second setof reference circuits.
 3. The memory device of claim 2, wherein thefirst multiplexer is to electrically couple one of the first set ofreference circuits to the second input port of the first sense amplifierduring a first time period, and wherein the second multiplexer is toelectrically couple one of the second set of reference circuits to thesecond input port of the second sense amplifier during the first timeperiod.
 4. The memory device of claim 3, wherein the first senseamplifier is to compare a characteristic of the multi-level cell with afirst reference characteristic of the one of the first set of referencecircuits during the first time period, and the second sense amplifier isto compare the characteristic of the multi-level cell with a secondreference characteristic of the one of the second set of referencecircuits during the first time period.
 5. The memory device of claim 4,further comprising: a sense controller coupled to the first senseamplifier and the second sense amplifier, the sense controller todetermine one or more bits of the multiple bits of data stored by themulti-level cell based at least in part on the comparisons from thefirst sense amplifier and the second sense amplifier during the firsttime period.
 6. The memory device of claim 5, wherein the sensecontroller is to select another one of the first set of referencecircuits and another one of the second set of reference circuits,according to the one or more bits of the multiple bits of data, andconfigure the first multiplexer and the second multiplexer according tothe selection.
 7. The memory device of claim 6, wherein the firstmultiplexer is to electrically couple the another one of the first setof reference circuits to the second input port of the first senseamplifier during a second time period, wherein the second multiplexer isto electrically couple the another one of the second set of referencecircuits to the second input port of the second sense amplifier duringthe second time period.
 8. The memory device of claim 7, wherein thefirst sense amplifier is to compare the characteristic of themulti-level cell with a third reference characteristic of the anotherone of the first set of reference circuits during the second timeperiod, and the second sense amplifier is to compare the characteristicof the multi-level cell with a fourth reference characteristic of theanother one of the second set of reference circuits during the secondtime period.
 9. The memory device of claim 8, wherein the sensecontroller is to determine additional one or more bits of the multiplebits of data stored by the multi-level cell based at least in part onthe comparisons from the first sense amplifier and the second senseamplifier during the second time period.
 10. A memory device comprising:a multi-level cell storing multiple bits of data; and a sensing devicecoupled to the multi-level cell, the sensing device including: a set ofsense amplifiers, each of the set of sense amplifiers including a firstinput port and a second input port, the first input port coupled to themulti-level cell, a set of reference circuits, and a set ofmultiplexers, each of the set of multiplexers coupled between the secondinput port of a corresponding one of the set of sense amplifiers and acorresponding subset of the set of reference circuits.
 11. The memorydevice of claim 10, wherein a first multiplexer of the set ofmultiplexers is to electrically couple one of a first subset of the setof reference circuits to a first sense amplifier of the set of senseamplifiers during a first time period, and wherein a second multiplexerof the set of multiplexers is to electrically couple one of a secondsubset of the set of reference circuits to a second sense amplifier ofthe set of sense amplifiers during the first time period.
 12. The memorydevice of claim 11, wherein the first sense amplifier is to compare acharacteristic of the multi-level cell with a first referencecharacteristic of the one of the first subset of the set of referencecircuits during the first time period, and the second sense amplifier isto compare the characteristic of the multi-level cell with a secondreference characteristic of the one of the second subset of the set ofreference circuits during the first time period.
 13. The memory deviceof claim 12, further comprising: a sense controller coupled to the setof sense amplifiers, the sense controller to determine one or more bitsof the multiple bits of data stored by the multi-level cell based atleast in part on the comparisons from the first sense amplifier and thesecond sense amplifier during the first time period.
 14. The memorydevice of claim 13, wherein the sense controller is to select anotherone of the first subset of the set of reference circuits and another oneof the second subset of the set of reference circuits, according to theone or more bits of the multiple bits of data, and configure the firstmultiplexer and the second multiplexer according to the selection. 15.The memory device of claim 14, wherein the first multiplexer is toelectrically couple the another one of the first subset of the set ofreference circuits to the second input port of the first sense amplifierduring a second time period, wherein the second multiplexer is toelectrically couple the another one of the second subset of the set ofreference circuits to the second input port of the second senseamplifier during the second time period.
 16. The memory device of claim15, wherein the first sense amplifier is to compare the characteristicof the multi-level cell with a third reference characteristic of theanother one of the first subset of the set of reference circuits duringthe second time period, and the second sense amplifier is to compare thecharacteristic of the multi-level cell with a fourth referencecharacteristic of the another one of the second subset of the set ofreference circuits during the second time period.
 17. The memory deviceof claim 16, wherein the sense controller is to determine additional oneor more bits of the multiple bits of data stored by the multi-level cellbased at least in part on the comparisons from the first sense amplifierand the second sense amplifier during the second time period.
 18. Amethod comprising: selecting a first reference circuit from a first setof reference circuits; selecting a second reference circuit from asecond set of reference circuits; determining one or more bits ofmultiple bits of data stored by a multi-level cell, based at least inpart on the first reference circuit and the second reference circuit;selecting a third reference circuit from the first set of referencecircuits, according to the determined one or more bits; selecting afourth reference circuit from the second set of reference circuits,according to the determined one or more bits; and determining additionalone or more bits of the multiple bits of data stored by the multi-levelcell, based at least in part on the third reference circuit and thefourth reference circuit.
 19. The method of claim 18, furthercomprising: comparing, by a first sense amplifier, a characteristic ofthe multi-level cell with a first reference characteristic of the firstreference circuit during a first time period; and comparing, by a secondsense amplifier, the characteristic of the multi-level cell with asecond reference characteristic of the second reference circuit duringthe first time period, wherein the one or more bits of the multiple bitsof data stored by the multi-level cell are determined based at least inpart on the comparisons during the first time period.
 20. The method ofclaim 19, further comprising: comparing, by the first sense amplifier,the characteristic of the multi-level cell with a third referencecharacteristic of the third reference circuit during a second timeperiod; comparing, by the second sense amplifier, the characteristic ofthe multi-level cell with a fourth reference characteristic of thefourth reference circuit during the second time period, and wherein theadditional one or more bits of the multiple bits of data stored by themulti-level cell are determined based at least in part on thecomparisons during the second time period.